There is no way to target or spot reduce fat. This is a common myth, people think if they do 10 million crunches it will shrink their belly fat. this is not true.
The simple truth is you need to
a. be in a caloric deficit you can use any web calculator, to figure out your maintenance is, and then eat 20-30% less than that.
b. eat a high amount of protien, about 1g per LBs of body weight. this should prevent your body from using your muscle mass as fuel instead of your fat stores.
c. progressive resistance training. (training close to failure and progressively increasing weight/volume) this should help you maintain your current level of muscle mass while burning
how long it takes for your body to start burning the belly fat will be determined by your genetics.